Tibial baseplate with asymmetric placement of fixation structures

ABSTRACT

An orthopaedic knee prosthesis is provided including a tibial baseplate component having a distal, bone-contacting surface with one or more fixation structures extending distally therefrom, in which the fixation structures are asymmetrically arranged within the baseplate periphery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under Title 35 U.S.C. §119(e) ofU.S. Provisional Patent Application Ser. No. 61/562,133 filed Nov. 21,2011, U.S. Provisional Patent Application Ser. No. 61/592,571 filed Jan.30, 2012, U.S. Provisional Patent Application Ser. No. 61/594,030 filedFeb. 2, 2012, and U.S. Provisional Patent Application Ser. No.61/621,369 filed Apr. 6, 2012, each entitled TIBIAL BASEPLATE WITHASYMMETRIC PLACEMENT OF FIXATION STRUCTURES and U.S. Provisional PatentApplication Ser. No. 61/592,574 filed Jan. 30, 2012 and U.S. ProvisionalPatent Application Ser. No. 61/621,374 filed Apr. 6, 2012, both entitledASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS. The entiredisclosures of all of the above-identified patent applications arehereby expressly incorporated by reference herein.

FIELD OF THE DISCLOSURE

The present disclosure relates to orthopaedic prostheses and, moreparticularly, to tibial baseplate components in a knee prosthesis.

BACKGROUND OF THE DISCLOSURE

Orthopaedic prostheses are commonly utilized to repair and/or replacedamaged bone and tissue in the human body. For a damaged knee, a kneeprosthesis may be implanted using a proximal tibial baseplate component,a tibial bearing component, and a distal femoral component. The tibialbaseplate component is affixed to a proximal end of the patient's tibia,which is typically resected to accept the baseplate component. Thefemoral component is implanted on a distal end of the patient's femur,which is also typically resected to accept the femoral component. Thetibial bearing component is placed between the tibial baseplatecomponent and the femoral component, and may be fixed or slidablycoupled to the tibial baseplate component.

The tibial baseplate component provides support for the tibial bearingcomponent. Forces generated by use of the knee prosthesis aretransferred through the tibial bearing component to the tibial baseplatecomponent, and ultimately to the tibia. In order to ensure long termperformance of the knee prosthesis, stable and firm securement of thetibial baseplate component to the proximal end of the patient's tibia isdesired.

SUMMARY

This application is related to U.S. Provisional Patent Application Ser.No. 61/562,133, filed Nov. 21, 2011 (Attorney Docket No. ZIM0913), toU.S. Provisional Patent Application Ser. No. 61/592,571, filed Jan. 30,2012 (Attorney Docket No. ZIM0913-01), and to U.S. Provisional PatentApplication Ser. No. 61/594,030, filed Feb. 2, 2012 (Attorney Docket No.ZIM0913-02), the entire disclosures of which are hereby expresslyincorporated by reference herein.

The present disclosure provides an orthopaedic knee prosthesis includinga tibial baseplate component having a distal, bone-contacting surfacewith one or more fixation structures extending distally therefrom, thefixation structures being asymmetrically arranged within the outerperiphery of the baseplate.

For designs utilizing a plurality of fixation pegs that extend distallyfrom the bone-contacting surface of the tibial baseplate, fixation pegsare asymmetrically arranged in opposite anterior/lateral andposterior/medial regions of the tibial baseplate, thereby maximizingdistance between the fixation pegs, avoiding overlap with theintramedullary canal, avoiding areas of low bone density, and avoidingcortical impingement by positioning the fixation pegs in regions ofcancellous bone.

For designs utilizing a single keel that extends distally from thebone-contacting surface of the tibial baseplate, the keel is medializedwith respect to the outer periphery of the tibial baseplate, where thedegree of medialization increases as prosthesis sizes growprogressively.

According to an embodiment thereof, the present disclosure provides atibial prosthesis system comprising: a first tibial baseplatecomprising: a first proximal surface; a first distal surface oppositethe first proximal surface, the first distal surface sized and shaped tosubstantially cover a proximal resected surface of a tibia; a firstmedial face; a first lateral face opposite the first medial face; afirst total width measured from the first medial face to the firstlateral face; and a first keel extending distally from the first distalsurface, the first keel spaced from the first medial face by a firstmedial distance and spaced apart from the first lateral face by a firstlateral distance; and a second tibial baseplate comprising: a secondproximal surface; a second distal surface opposite the second proximalsurface, the second distal surface sized and shaped to substantiallycover a proximal resected surface of a tibia; a second medial face; asecond lateral face opposite the second medial face; a second totalwidth measured between the second medial face and the second lateralface, the second total width differing from the first total widthwhereby the first and second tibial baseplates comprise unique nominalsizes; and a second keel extending distally from the second distalsurface, the second keel spaced apart from the second medial face by asecond medial distance and spaced apart from the second lateral face bya second lateral distance, a first ratio of the first medial distance tothe first total width differing from a second ratio of the second medialdistance to the second total width.

According to another embodiment thereof, the present disclosure providesa tibial baseplate configured for implantation upon a patient's proximaltibia, the tibial baseplate comprising: a medial compartment; a lateralcompartment opposite the medial compartment; a proximal surface; adistal surface opposite the proximal surface, the distal surface sizedand shaped to substantially cover the patient's proximal tibia; an outerperiphery cooperatively defined by an anterior face, a medial face, alateral face, and at least one posterior face; a first,anterior-posterior axis located between the medial face and the lateralface and intersecting the anterior face, the first axis extendingcentrally between the medial and lateral compartments throughout itslength; a plurality of fixation pegs extending distally from the distalsurface, each of the plurality of fixation pegs being positioned inwardof the outer periphery for implantation into the patient's proximaltibia, the plurality of fixation pegs comprising: a medial fixation peglocated at the medial compartment; and a lateral fixation peg located atthe lateral compartment, the lateral fixation peg being positioned moreanteriorly than each other fixation peg among the plurality of fixationpegs.

According to yet another embodiment thereof, the present disclosureprovides a tibial baseplate configured for implantation upon a patient'sproximal tibia, the tibial baseplate comprising: a medial compartment; alateral compartment opposite the medial compartment; a proximal surface;a distal surface opposite the proximal surface, the distal surface sizedand shaped to substantially cover the patient's proximal tibia; an outerperiphery cooperatively defined by an anterior face, a medial face, alateral face, and at least one posterior face; at most one medialfixation peg associated with the medial compartment, the medial fixationpeg extending distally from the distal surface and positioned forimplantation into the patient's proximal tibia; and at most one lateralfixation peg associated with the lateral compartment, the lateralfixation peg extending distally from the distal surface and positionedfor implantation into the patient's proximal tibia, the lateral fixationpeg being located closer to the anterior face than the medial fixationpeg.

According to still another embodiment thereof, the present disclosureprovides a tibial baseplate configured for implantation upon a patient'sproximal tibia, the tibial baseplate comprising: a medial compartment; alateral compartment opposite the medial compartment; a proximal surface;a distal surface opposite the proximal surface, the distal surface sizedand shaped to substantially cover the patient's proximal tibia; an outerperiphery cooperatively defined by an anterior face, a medial face, alateral face, and at least one posterior face; a first,anterior-posterior axis located between the medial face and the lateralface and intersecting the anterior face, the first axis extendingcentrally between the medial and lateral compartments throughout itslength; a first fixation peg extending distally from the distal surface,the first fixation peg being inset from the outer periphery forimplantation into the patient's proximal tibia, the first fixation pegbeing medially spaced from the first axis by a first distance; and asecond fixation peg extending distally from the distal surface, thesecond fixation peg being inset from the outer periphery forimplantation into the patient's proximal tibia, the second fixation pegbeing laterally spaced from the first axis by a second distance, thesecond distance less than the first distance.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and advantages of thisdisclosure, and the manner of attaining them, will become more apparentand the invention itself will be better understood by reference to thefollowing description of embodiments of the invention taken inconjunction with the accompanying drawings, wherein:

FIG. 1 is a proximal plan view of a tibial baseplate made in accordancewith the present disclosure, the baseplate having a lateral fixation pegand a medial fixation peg, the baseplate shown implanted upon theresected proximal surface of a patient's tibia, the baseplate shownhaving an asymmetric outer periphery in solid lines and a symmetricouter periphery in phantom;

FIG. 2A is a first, distal plan view of the baseplate of FIG. 1, showingmedial/lateral positioning of the fixation pegs and the overall medialbias thereof;

FIG. 2B is a second, distal plan view of the baseplate of FIG. 1 similarto FIG. 2A, showing anterior/posterior positioning of the fixation pegs;

FIG. 2C is a third distal plan view of the baseplate of FIG. 1 similarto FIGS. 2A and 2B, also showing anterior/posterior positioning of thefixation pegs;

FIG. 3 is a lateral elevational view of the baseplate of FIG. 1;

FIG. 4 is a distal plan view of an alternative baseplate;

FIG. 5 is a distal plan view of another alternative baseplate;

FIG. 6 is a graph illustrating the anterior/posterior positioning of thefixation pegs of FIGS. 1-3 across a range of prosthesis sizes;

FIG. 7 is a posterior perspective view of the baseplate of FIG. 1, shownwith a tibial bearing component mounted thereon;

FIG. 8 is a proximal plan view of another tibial baseplate made inaccordance with the present disclosure, the baseplate having a singlefixation keel for fixation to the patient's tibia;

FIG. 9A is an anterior elevational view of the baseplate of FIG. 8;

FIG. 9B is another anterior elevational view of the baseplate of FIG. 8;

FIG. 10 is a graph illustrating the medialization of the fixation keelof FIGS. 8 and 9 across a range of prosthesis sizes;

FIG. 11 is another graph illustrating the medialization of the fixationkeel of FIGS. 8 and 9 across a range of prosthesis sizes;

FIG. 12 is a graph illustrating the anterior/posterior positioning ofthe fixation keel of FIGS. 8 and 9 across a range of prosthesis sizes;and

FIG. 13 is a distal plan view of another baseplate similar to thebaseplate shown in FIGS. 2A-2C, but having another lateral fixation pegand another medial fixation peg; and

FIG. 14 is a perspective view of a posterior-stabilized femoralcomponent in accordance with the present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary embodiments of the invention and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION

The present disclosure provides a tibial baseplate component for a kneeprosthesis including asymmetrically arranged distal fixation structureswhich promote secure and stable long term fixation of the tibialbaseplate to a patient's proximal tibia.

In order to prepare the tibia and femur for receipt of a knee jointprosthesis of the present disclosure, any suitable methods orapparatuses for preparation of the knee joint may be used. The surgicalprocedure may involve, for example, forming an incision in the patient'sskin near the knee joint, resecting the distal end of the patient'sfemur (not shown), and resecting the proximal end of the patient's tibiaT (FIG. 1). Resecting the proximal end of the patient's tibia T (FIG.1), in particular, may involve guiding a saw blade through anappropriate cutting guide slot to form a substantially planar resectedsurface S of tibia T, as shown in FIG. 1.

Exemplary surgical procedures and associated surgical instruments aredisclosed in Zimmer's “LPS-Flex Fixed Bearing Knee, Surgical Technique”bearing copyright dates of 2004, 2007 and 2008, “NexGen” Complete KneeSolution, Surgical Technique for the CR-Flex Fixed Bearing Knee” bearinga copyright date of 2003, “NexGen® Complete Knee SolutionExtramedullary/Intramedullary Tibial Resector, Surgical Technique”bearing copyright dates of 2000, 2008 and 2009, “NexGen® TrabecularMetal™ Monoblock Tibial Components, Surgical Technique Addendum,”bearing copyright dates of 2005 and 2007, “NexGen® Trabecular Metal™Tibial Tray, Surgical Technique,” bearing copyright dates of 2007 and2009, and “Trabecular Metal™ Monoblock Tibial Components,” bearing acopyright date of 2007 (collectively, the “Zimmer Surgical Techniques”),the entire disclosures of which are hereby expressly incorporated hereinby reference, copies of which are submitted on even date herewith in anInformation Disclosure Statement.

As used herein, “proximal” refers to a direction generally toward thetorso of a patient, and “distal” refers to the opposite direction ofproximal (i.e., away from the torso of a patient). “Anterior” refers toa direction generally toward the front of a patient or knee, and“posterior” refers to the opposite direction of anterior (i.e., towardthe back of the patient or knee). “Lateral” refers to a directiongenerally away from the middle of the patient and the sagittal plane,and “medial” refers to the opposite direction of lateral (i.e., towardthe middle of the patient and the sagittal plane). When referring to oneof the patient's knees, “lateral” refers to the direction generally awayfrom the other knee, and “medial” refers to the direction generallytoward the other knee.

These anatomical regions are labeled in certain drawings for clarity. InFIG. 1, for example, the anterior region of tibia T is labeled “A,” theposterior region of tibia T is labeled “P,” the lateral region of tibiaT is labeled “L,” and the medial region of tibia T is labeled “NM.”Therebetween and moving in a clock-wise direction, the anterior/lateralregion of tibia T is labeled “AL,” the posterior/lateral region of tibiaT is labeled “PL,” the posterior/medial region of tibia T is labeled“PM,” and the anterior/medial region of tibia T is labeled “AM.” The AL,PL, PM, and AM regions can be described as dividing tibia T into fourcorners or quadrants. These labels are referenced throughout thefollowing paragraphs.

The embodiments shown and described herein illustrate components for aright knee prosthesis. Right and left knee prosthesis configurations aregenerally mirror images of one another about a sagittal plane. Thus, itwill be appreciated that the aspects of the prosthesis described hereinfor a right knee configuration are equally applicable to a left kneeconfiguration.

1. Tibial Baseplate

Referring now to FIG. 1, tibial baseplate 10 is shown disposed atop aproximal resected surface S of a patient's tibia T. The upper orproximal surface 11 of baseplate 10 is shown in FIG. 1. This proximalsurface 11 of baseplate 10 is configured to receive a tibial bearingcomponent 53 (FIG. 7) in a fixed or a sliding relationship, for example.To arrange baseplate 10 and the tibial bearing component 53 in a fixedrelationship, the tibial bearing component 53 may be adhered to,mechanically fastened to, molded directly onto (as discussed furtherbelow), or otherwise fixedly coupled to baseplate 10. The illustrativebaseplate 10 includes a raised rim 13 around proximal surface 11 toreceive, surround, and hold the tibial bearing component 53 therein, butit is contemplated that other structures may be provided on baseplate 10to receive and hold the tibial bearing component 53 on baseplate 10. Inturn, tibial bearing component 53 is configured to interact with thepatient's distal femur or a prosthetic femoral component, such asfemoral component 70 shown in FIG. 14 and described below.

Baseplate 10 may be partially or entirely constructed of a highly porousbiomaterial. A highly porous biomaterial is useful as a bone substituteand as cell and tissue receptive material. A highly porous biomaterialmay have a porosity as low as 55%, 65%, or 75% or as high as 80%, 85%,or 90%. An example of such a material is produced using TrabecularMetal™ Technology generally available from Zimmer, Inc., of Warsaw, Ind.Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material may beformed from a reticulated vitreous carbon foam substrate which isinfiltrated and coated with a biocompatible metal, such as tantalum, bya chemical vapor deposition (“CVD”) process in the manner disclosed indetail in U.S. Pat. No. 5,282,861 to Kaplan, the entire disclosure ofwhich is expressly incorporated herein by reference. In addition totantalum, other metals such as niobium, or alloys of tantalum andniobium with one another or with other metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%, 85%, or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of baseplate 10 to the patient's bone.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an improved matrix for boneingrowth and mineralization.

Bearing component 53 may be molded directly onto baseplate 10,specifically proximal surface 11 of baseplate 10. If baseplate 10 isconstructed of a highly porous biomaterial, as discussed above, thematerial that is used to construct bearing component 53 (e.g.,polyethylene) may interdigitate into the pores of baseplate 10 duringthe molding process. The pores may be located at and beneath proximalsurface 11 of baseplate 10, so the resulting molded bearing component 53may also be located at and beneath proximal surface 11 of baseplate 10.The resulting structure may be a monoblock component having a strong,wear-resistant connection between baseplate 10 and bearing component 53,especially along proximal surface 11 of baseplate 10.

Baseplate 10 includes outer periphery 12, which may be visible in a topplan view (FIG. 1) or a bottom plan view (FIGS. 2A-2C) with baseplate 10positioned in a generally transverse anatomical plane. As shown in FIG.1, outer periphery 12 is cooperatively defined by anterior face 18,posterior/lateral face 20, posterior/medial face 22, PCL cutout area 24,lateral face 62, and medial face 60. Each of these surfaces is describedfurther below.

Baseplate 10 also includes lateral compartment 14, medial compartment16, and interior compartment 17 therebetween. Lateral compartment 14 andmedial compartment 16 are separated by an anterior-posterior home axisA_(H), which is discussed further below. Because FIG. 1 is a proximalview of the patient's right tibia T, lateral compartment 14 of baseplate10 is located on the right side of FIG. 1 and medial compartment 16 ofbaseplate 10 is located on the left side of FIG. 1.

With bearing component 53 in place against baseplate 10 (FIG. 7) toarticulate with adjacent femoral component 70, for example, lateralcompartment 14 of baseplate 10 will be positioned generally beneathlateral condyle 74 of femoral component 70 to support and articulatewith lateral condyle 74, and medial compartment 16 of baseplate 10 willbe positioned generally beneath medial condyle 72 of femoral component70 to support medial condyle 72. Tibial bearing component 53 (FIG. 7)may be disposed between medial and lateral condyles 72, 74 of femoralcomponent 70 and medial and lateral compartments 16, 14 to provide alow-friction articular interface, as described below. In theillustrative embodiment, femoral component 70 includes cam 76 adapted toarticulate with a spine of a tibial bearing component, e.g., spine 58 oftibial bearing component 53 (FIG. 7). However, it is contemplated thatfemoral component 70 may omit spine 76 to provide an uninterrupted spacebetween medial and lateral condyles 72, 74 in some prosthesis designs.

Anterior face 18 of the illustrative baseplate 10 is disposed anteriorlyon periphery 12 of baseplate 10 (i.e., in the A region of tibia T),Anterior face 18 is generally centrally located between lateral andmedial compartments 14, 16. More specifically, as shown in FIG. 1,anterior face 18 includes a linear or flat portion 18 a that isgenerally centrally located between lateral and medial compartments 14,16. In this illustrated embodiment, flat portion 18 a of anterior face18 defines the anterior-most extent of baseplate 10.

Posterior/lateral face 20 of the illustrative baseplate 10 is disposedgenerally opposite anterior face 18 in the posterior region of lateralcompartment 14 (i.e., near the PL region of tibia T). Posterior/medialface 22 of the illustrative baseplate 10 is disposed generally oppositeanterior face 18 in the posterior region of medial compartment 16 (i.e.,near the PM region of tibia T). The PCL cutout area 24 is disposedbetween posterior/lateral face 20 and posterior/medial face 22 (i.e.,near the P region of tibia T). From both posterior/lateral face 20 andposterior/medial face 22, the PCL cutout area 24 extends generallyanteriorly until reaching apex 24 a.

Lateral face 62 of the illustrative baseplate 10 is disposed laterallyof lateral compartment 14 on periphery 12 of baseplate 10 (i.e., nearthe L region of tibia T). Medial face 60 of the illustrative baseplate10 is located medially of medial compartment 16 on periphery 12 ofbaseplate 10 (i.e., near the M region of tibia. T).

2. Home Axis and Other Reference Axes of Tibial Baseplate

In the context of patient anatomy, such as tibia T described herein,“home axis” A_(H) of tibia T′ extends anteriorly from a posterior pointP_(P) on tibia T′ to an anterior point P_(A) on tibia T. The posteriorpoint P_(P) and the anterior point P_(A) of tibia T are discussedfurther below.

The posterior point P_(P) is generally disposed in the area where thepatient's posterior cruciate ligament (PCL) attaches to tibia T. Morespecifically, the posterior point P_(P) is generally disposed at thegeometric center of the attachment between the patient's PCL and tibiaT. The patient's PCL typically attaches to tibia T in two ligament“bundles,” the first bundle having a more anterolateral attachmentlocation and the second bundle having a more posteromedial attachmentlocation. In FIG. 1, the posterior point P_(P) is shown at the geometriccenter of the first bundle. It is also within the scope of the presentdisclosure that the posterior point P_(P) may be located at thegeometric center of the second bundle or at the geometric center of thefirst and second bundles, together.

The anterior point P_(A) is disposed on the patient's anterior tibialtubercle B. In FIG. 1, the anterior point P_(A) is medially spaced fromthe tubercle midpoint B_(A) (at marking ½) by an amount equal to ⅙ ofthe overall medial/lateral tubercle width B_(W) (which spans betweenmarkings 0 and 1). Stated another way, the anterior point P_(A) islaterally spaced from the tubercle medial end B_(ME) (at marking 0) byan amount equal to ⅓ of the overall medial/lateral tubercle width B_(W)(which spans between markings 0 and 1), such that the anterior pointP_(A) lies on the “medial third” of the anterior tibial tubercle B (atmarking ⅓).

In the context of a prosthesis, such as tibial baseplate 10 describedherein, “home axis” An of baseplate 10 refers to an anterior-posteriorextending axis of baseplate 10 that aligns with home axis A_(H) of tibiaT upon implantation of baseplate onto resected surface S of tibia T in aproper rotational and spatial orientation (as shown in FIG. 1).According to an exemplary embodiment of the present disclosure, and asshown in FIG. 1, home axis A_(H) of baseplate 10 is centrally locatedbetween the inner-most portion of lateral compartment 14 and theinner-most portion of medial compartment 16 of baseplate 10 throughoutits length. In other words, home axis A_(H) of baseplate 10 isequidistant from the inner-most portion of lateral compartment 14 andthe inner-most portion of medial compartment 16 of baseplate 10 todivide the interior compartment 17 therebetween into substantially equalhalves.

In the illustrative embodiment of FIG. 1, home axis A_(H) of baseplate10 bisects anterior face 18 of baseplate 10 (which is located anteriorlyon periphery 12 of baseplate 10) and is generally perpendicular to flatportion 18 a of anterior surface 18, Also, home axis A_(H) of baseplate10 bisects PCL cutout area 24 of baseplate 10 (which is locatedposteriorly on periphery 12 of baseplate 10) and is generallyperpendicular to apex 24 a of PCL cutout area 24. It is contemplatedthat home axis A_(H) of baseplate 10 may be oriented to other featuresof baseplate 10, it being understood that proper alignment andorientation of baseplate 10 upon resected surface S of tibia T willposition home axis A_(H) of baseplate 10 coincident with home axis A_(H)of tibia T.

The home axes A_(H) of tibia T and baseplate 10 are further described inU.S. Patent Application Publication No. 2012/0022659, filed Jul. 22,2011, entitled “ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS,” theentire disclosure of which is hereby expressly incorporated herein byreference.

A pair of reference axes 26, 28 is presented in FIG. 1. A firstreference axis 26 extends diagonally across baseplate 10 from theback-left PM region of tibia T to the front-right AL region of tibia T,intersecting home axis Au to define a first angle α with home axisA_(H), as shown in FIG. 1. A second reference axis 28 extends diagonallyacross baseplate 10 and perpendicularly to the first axis 26 from theback-right PL region of tibia T to the front-left AM region of tibia T,intersecting home axis A_(H) to define a second angle β with home axisA_(H), as shown in FIG. 1. The first and second angles α and β are eachapproximately 45 degrees such that, when combined, the first and secondangles α and β together total approximately 90 degrees.

The first and second reference axes 26, 28 illustratively intersect oneanother and home axis A_(H) at a common point X within periphery 12 ofbaseplate 10. According to an exemplary embodiment of the presentdisclosure, point X is generally centered within periphery 12 ofbaseplate 10 to maximize the aggregated extent of each reference axis26, 28 that is located within periphery 12 of baseplate whilemaintaining the desired first and second angles α and β, as discussedabove. Point X is illustratively positioned along home axis A_(H)between flat portion 18 a of anterior face 18 and apex 24 a of PCLcutout area 24.

Illustratively, a medial-lateral axis 50 also extends through point X ina direction perpendicular to home axis A_(H). Together, themedial-lateral axis 50 (e.g., the x-axis) and the anterior-posteriorhome axis A_(H) (e.g. the y-axis) cooperate to define a componentcoordinate system (e.g., an x-y coordinate system) useful forquantifying and identifying certain features of baseplate 10.

3. Shape of Outer Periphery of Tibial Baseplate

According to an exemplary embodiment of the present disclosure, and asshown in FIG. 1, baseplate 10 has an asymmetric outer periphery 12. Theasymmetric outer periphery 12 may be designed to closely match thecorresponding periphery of resected surface S of tibia T. In theillustrated embodiment of FIG. 1, for example, medial compartment 16 islarger than lateral compartment 14. Medial compartment 16 is wider thanlateral compartment 14, so medial face 60 is spaced further apart fromthe anterior-posterior home axis A_(H) than lateral face 62. Medialcompartment 16 is also deeper than lateral compartment 14, soposterior/medial face 22 is spaced further apart posteriorly from themedial-lateral axis 50 than posterior/lateral face 20. For at leastthese reasons, the outer periphery 12 of baseplate 10 is asymmetric.

The asymmetric shape of baseplate 10 is further described in U.S. PatentApplication Publication No. 2012/0022659, filed Jul. 22, 2011, entitled“ASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS,” the entiredisclosure of which is hereby expressly incorporated herein byreference.

It is also within the scope of the present disclosure that baseplate 10may have a symmetric outer periphery 212, as shown in phantom in FIG. 1.In this embodiment, lateral compartment 14 and medial compartment 16 arethe same shape and size. Lateral compartment 14 and medial compartment16 are the same width, so lateral face 62 and the modified medial face260 (shown in phantom) are equidistant from the anterior-posterior homeaxis A_(H). In this manner, an anterior-posterior axis of symmetrythrough outer periphery 212 of symmetric baseplate 10 may overlay “homeaxis” A_(H) and may serve as a reference for lateral compartment 14,medial compartment 16, lateral face 62, medial face 260, lateral andfixation pegs 30, 32 (described below) and other components of baseplate10. Thus, in addition to being centered within interior compartment 17between lateral compartment 14 and medial compartment 16 of thesymmetric embodiment of baseplate 10, the anterior-posterior home axisA_(H) would also be centered between lateral face 62 and the modifiedmedial face 260 (shown in phantom). Lateral compartment 14 and medialcompartment 16 also define a common anterior/posterior depth, soposterior/lateral face 20 and the modified posterior/medial face 222(shown in phantom) are equidistant from the medial-lateral axis 50.Generally, a symmetric outer periphery 212 allows the same baseplate 10to be implanted onto either a patient's right tibia or left tibia.

4. Fixation Pegs

Referring next to FIGS. 2A-2C and 3, the underside or distal surface 34of baseplate 10 is shown. Distal surface 34 is the surface whichcontacts resected surface S of tibia T (FIG. 1) after implantation ofbaseplate 10. As shown in FIG. 3, distal surface 34 is located oppositeproximal surface 11. Baseplate 10 includes a plurality of fixationstructures, illustratively lateral fixation peg 30 and medial fixationpeg 32, that extend distally from distal surface 34 and into tibia T(FIG. 1).

Each fixation peg 30, 32 is inset from outer periphery 12 of baseplate10. Each fixation peg 30, 32 may have a minimum inset distance 39 (FIG.2A) that exceeds 0 mm, such as 1 mm, 3 mm, 5 mm, or more, for example.For purposes of the present disclosure, and as shown in FIG. 2A, theminimum inset distance 39 is the smallest distance measured betweenouter periphery 12 of baseplate 10 and the outer perimeter of eachfixation peg 30, 32.

According to an exemplary embodiment of the present disclosure, fixationpegs 30, 32 of baseplate 10 are constructed of a highly porousbiomaterial, such as the above-described porous tantalum material.Distal surface 34 of baseplate 10 may also be constructed of a highlyporous biomaterial. With distal surface 34 of baseplate 10 restingagainst resected surface S of tibia T and fixation pegs 30, 32 ofbaseplate 10 extending distally into tibia T, the highly porousbiomaterial may provide a matrix into which cancellous bone may grow toprovide fixation of baseplate 10 to tibia T.

As shown in FIG. 3, the illustrative fixation pegs 30, 32 are hexagonalin cross-section near distal surface 34 of baseplate 10. As fixationpegs 30, 32 continue extending distally away from distal surface 34 ofbaseplate 10, fixation pegs 30, 32 transition to a circularcross-section. The hexagonal to circular transition of fixation pegs 30,32 is also evident in FIGS. 2A-2C. In FIG. 1, by contrast, each fixationpeg 30, 32 is represented by a phantom circle to schematically show thegeneral location of each fixation peg 30, 32, not necessarily the sizeor shape of each fixation peg 30, 32. Exemplary fixation pegs 30, 32 areshown at pages 16-19 of the “Zimmer® Tibial Baseplate, Pocket GuideUnited States Version,” the entire disclosure of which is herebyexpressly incorporated herein by reference, a copy of which is submittedon even date herewith in an Information Disclosure Statement.

According to an exemplary embodiment of the present disclosure, and asdiscussed further below, lateral and medial fixation pegs 30, 32 areasymmetrically arranged on distal surface 34 of baseplate 10. In oneexemplary embodiment, fixation pegs 30, 32 are asymmetrically arrangedabout the anterior-posterior home axis A_(H), such that theanterior-posterior home axis A_(H) is not arm axis of symmetry offixation pegs 30, 32. In another embodiment, fixation pegs 30, 32 areasymmetrically arranged about the medial-lateral axis 50, such that themedial-lateral axis 50 is not an axis of symmetry of fixation pegs 30,32. In yet another embodiment, fixation pegs 30, 32 are asymmetricallyarranged about both the anterior-posterior home axis A_(H) and themedial-lateral axis 50, such that neither the anterior-posterior homeaxis A_(H) nor the medial-lateral axis 50 is an axis of symmetry offixation pegs 30, 32.

5. Anterior/Lateral (AL) and Posterior/Medial (PM) Positioning ofFixation Pegs

Returning now to FIG. 1, lateral fixation peg 30 in lateral compartment14 of baseplate 10 is positioned anteriorly relative to themedial-lateral axis 50 and anteriorly of medial fixation peg 32. Thus,lateral fixation peg 30 is more generally positioned in the AL region oftibia T while being substantially distanced from the PL region of tibiaT. The AL bias of lateral fixation peg 30 is evident in FIG. 1, becausefrom the center point X, the first axis 26 extends toward the AL regionand approaches or even intersects lateral fixation peg 30, while thesecond axis 28 extends toward the PL region extends further away fromlateral fixation peg 30.

In the medial compartment 16 of baseplate 10, medial fixation peg 32 ispositioned posteriorly relative to the medial-lateral axis 50 andposteriorly of lateral fixation peg 30. Thus, medial fixation peg 32 ismore generally positioned in the PM region of tibia T while beingsubstantially distanced from the AM region of tibia T. The PM bias ofmedial fixation peg 32 is evident in FIG. 1, because from the centerpoint X, the first, axis 26 extends toward the PM region and approachesor even intersects medial fixation peg 32, while the second axis 28extends toward the AM region and travels away from medial fixation peg32. In this exemplary embodiment, both fixation pegs 30, 32 aregenerally positioned along the same first reference axis 26 which spansthe PM and AL regions.

An alternative baseplate 10′ is shown in FIG. 4 for contrast. Outerperiphery 12′ of the alternative baseplate 10′ of FIG. 4 is generallythe same as outer periphery 12 of baseplate 10 (shown in solid lines inFIG. 1)—both are asymmetric in shape. However, unlike fixation pegs 30,32 of FIGS. 2A-2C, which are located on opposite sides of themedial-lateral axis 50, fixation pegs 30′, 32′ of FIG. 4 are alignedalong and intersect with medial-lateral axis 50′. With respect tobaseplate 10′, both the anterior-posterior home axis A_(H)′ and themedial-lateral axis 50′ are axes of symmetry for fixation pegs 30′, 32′,such that fixation pegs 30′, 32′ may be said to be symmetricallyoriented with respect to the component coordinate system.

Another alternative baseplate 10″ is shown in FIG. 5 for contrast. Outerperiphery 12″ of the alternative baseplate 10″ of FIG. 5 is generallythe same as outer periphery 212 of baseplate 10 (shown in phantom inFIG. 1)—both are symmetric in shape. Lateral compartment 14″ of thealternative baseplate 10″ is generally the same size and shape as medialcompartment 16″ of the alternative baseplate 10″. Therefore, theanterior-posterior home axis A_(H)″ is an axis of symmetry for outerperiphery 12″ of baseplate 10″. Like fixation pegs 30′, 32′ of FIG. 4,fixation pegs 30″, 32″ of FIG. 5 are aligned along and intersect withmedial-lateral axis 50″. With respect to baseplate 10″, both theanterior-posterior home axis A_(H)″ and the medial-lateral axis 50″ areaxes of symmetry for fixation pegs 30″, 32″, such that that fixationpegs 30″, 32″ may be said to be symmetrically oriented with respect tothe component coordinate system.

Returning again to FIG. 1, the asymmetric positioning of lateral andmedial fixation pegs 30, 32 near opposite AL and PM corners orquadrants, respectively, allows fixation pegs 30, 32 to be widely spacedapart across distal surface 34 of baseplate 10. Advantageously, thiswide spacing facilitates avoidance of the anatomic intramedullary canalof tibia T upon implantation (which may be located near the intersectionpoint X), particularly where baseplate 10 is used for a small-staturepatient. By avoiding placement of fixation pegs 30, 32 within theintramedullary canal of tibia T, the associated areas of low bonedensity are avoided and, instead, fixation pegs 30, 32 may be implantedinto areas of higher bone density, thereby promoting firm and stablelong-term fixation of tibial baseplate 10 to tibia T. If fixation pegs30, 32 are constructed of a highly porous biomaterial, as discussedabove, this firm and stable long-term fixation may be achieved bycancellous bone growth into the porous fixation pegs 30, 32. Alsoadvantageously, the wide spacing between fixation pegs 30, 32 encouragesbone ingrowth therebetween. By contrast, if fixation pegs 30, 32 are tooclose together, there may not be enough space for bone to growtherebetween.

Also, the asymmetric arrangement of lateral and medial fixation pegs 30,32 on opposite sides of the medial-lateral axis 50 may enhance thetorsional stability of baseplate 10 when implanted upon tibia T (FIG.1). During normal use, a significant portion of the forces generated onbaseplate 10 are directed anteriorly or posteriorly. Activities whichprimarily generate such anteriorly-directed or posteriorly-directedforces include walking, running, squatting, and climbing stairs, forexample. As shown in FIG. 3, such anteriorly-directed andposteriorly-directed forces give rise to anterior torsional momentsM_(A) and posterior torsional moments M_(P), respectively, which urgerotation of baseplate 10 anteriorly and posteriorly about themedial-lateral axis 50. Having lateral and medial fixation pegs 30, 32positioned on opposite sides of the medial-lateral axis 50 (i.e., theaxis of rotation), as illustrated in FIG. 3 and discussed in detailabove, presents greater resistance to such rotation.

Furthermore, positioning lateral and medial fixation pegs 30, 32 in theAL and PM regions of tibia T, rather than the PL and AM regions of tibiaT, may avoid impingement of pegs 30, 32 on adjacent cortical bone uponimplantation of baseplate 10. Advantageously, the AL and PM regions oftibia T (where fixation pegs 30, 32 are located) are typically populatedwith substantial areas of cancellous bone, thereby promoting firm andstable long-term fixation of tibial baseplate 10 to tibia T andpromoting bone ingrowth. By contrast, the PL and AM regions of tibia T(where fixation pegs 30, 32 are not located) are typically populatedwith substantial areas of cortical bone. By avoiding the PL and AMregions of tibia T, the potential for impingement of fixation pegs 30,32 upon cortical bone is minimized.

6. Lateral/Medial Positioning of Fixation Pegs

Because lateral fixation peg 30 extends from lateral compartment 14 andmedial fixation peg 32 extends from medial compartment 16, as discussedabove, lateral fixation peg 30 can be said to be positioned “morelaterally” on distal surface 34 of baseplate 10 than medial fixation peg32. Similarly, medial fixation peg 32 is positioned “more medially” ondistal surface 34 of baseplate 10 than lateral fixation peg 30. Thus, asshown in FIG. 2A, fixation pegs 30, 32 are spaced apart by amedial-lateral separation distance 36. For purposes of the presentdisclosure, the medial-lateral separation distance 36 is measured oncenter between fixation pegs 30, 32 along a direction perpendicular tohome axis A_(H) and parallel to medial-lateral axis 50 (FIG. 2B). In anexemplary embodiment, the medial-lateral separation distance 36 isbetween 20 mm and 55 mm, with smaller separation distances 36corresponding to smaller nominal prosthesis sizes, and larger separationdistances 36 corresponding to larger nominal prosthesis sizes.

According to an exemplary embodiment of the present disclosure, lateralfixation peg 30 and/or medial fixation peg 32 are medially biased intheir respective compartments 14, 16. In lateral compartment 14, theillustrative lateral fixation peg is medially biased toward home axisA_(H). In medial compartment 16, the illustrative medial fixation peg 32is medially biased away from home axis Ai. The medial bias of fixationpegs 30, 32, is evident in FIG. 2A, for example, where central peg axis38 (which is centered along the medial-lateral separation distance 36between fixation pegs 30, 32) is medially biased toward medialcompartment 16 and away from home axis A_(H). Because central peg axis38 is centered along medial-lateral separation distance 36, central pegaxis 38 divides medial-lateral separation distance 36 into equalhalves—one half being located between lateral fixation peg 30 andcentral peg axis 38 and the other half being located between medialfixation peg 32 and central peg axis 38.

If fixation pegs 30, 32 were equally spaced apart from home axis A_(H),central peg axis 38 would coincide with home axis A_(H). However, inFIG. 2A, pegs 30, 32 are not equally spaced apart from home axis A_(H).Instead, lateral fixation peg 30 is located closer to home axis Au thanmedial fixation peg 32. As a result, central peg axis 38 betweenfixation pegs 30, 32 is medially spaced or offset toward medialcompartment 16 and away from home axis A_(H) by offset distance 40.Therefore, fixation pegs 30, 32 may be said to be asymmetrically,medially biased relative to home axis A_(H). In an exemplary embodiment,offset distance 40 is between 3 mm and 6 mm. Smaller prosthesis sizesmay have smaller values for offset distance 40, while larger prosthesissizes may have larger values for offset distance 40,

7. Anterior/Posterior Positioning of Fixation Pegs

As discussed above, lateral fixation peg 30 is positioned relativelymore anteriorly on distal surface 34 of baseplate 10 than medialfixation peg 32. Stated differently, medial fixation peg 32 ispositioned relatively more posteriorly on distal surface 34 of baseplate10 than lateral fixation peg 30. Thus, as shown in FIG. 2B, pegs 30, 32are spaced apart by an anterior-posterior separation distance 42. Forpurposes of the present disclosure, the anterior-posterior separationdistance 42 is measured on center between fixation pegs 30, 32 along adirection parallel to home axis A_(H). In an exemplary embodiment, theanterior-posterior separation distance 42 is between 5 mm and 11 mm,with smaller separation distances 42 corresponding to smaller prosthesissizes, and larger separation distances 42 corresponding to largerprosthesis sizes.

The alternative baseplates 10′, 10″ of FIGS. 4 and 5 are provided forcontrast. Because lateral and medial fixation pegs 30′, 32′ of thealternative baseplate 10′ of FIG. 4, for example, are aligned in ananterior-posterior direction, lateral and medial fixation pegs 30′, 32′lack an anterior-posterior separation distance analogous to theanterior-posterior separation distance 42 of FIG. 2B. Or stateddifferently, lateral and medial fixation pegs 30′, 32′ have ananterior-posterior separation distance equal to 0 mm. Similarly, lateraland medial fixation pegs 30″, 32″ of the alternative baseplate 10″ ofFIG. 5 are aligned in an anterior-posterior direction and, therefore,have an anterior-posterior separation distance equal to 0 mm.

Turning now to FIG. 2C, another way of quantifying theanterior/posterior asymmetry of fixation pegs 30, 32 is by contrastingtheir different positions relative to a common reference marker. In FIG.2C, for example, the common reference marker is flat portion 18 a ofanterior face 18 of baseplate 10, with measurements being takenposteriorly therefrom in a direction parallel to home axis A_(H).Lateral fixation peg 30 is spaced posteriorly from anterior face 18 by arelatively smaller lateral peg distance 46, while medial fixation peg 32is spaced posteriorly from anterior face 18 by a relatively largermedial peg distance 48. The lateral anterior/posterior depth 44 oflateral compartment 14 of baseplate 10 is also shown being measured fromanterior face 18 to posterior/lateral face 20 of baseplate 10, and thislateral anterior/posterior depth 44 exceeds both peg distances 46, 48.Similarly, medial anterior/posterior depth 45 of medial compartment 16of baseplate is also shown being measured from anterior face 18 toposterior/medial face 22 of baseplate 10, and medial anterior/posteriordepth 45 exceeds both peg distances 46, 48, as well as lateralanterior/posterior depth 44. If baseplate 10 had a symmetric outerperiphery 212 (shown in phantom in FIG. 1) instead of the asymmetricouter periphery 12 of FIG. 2C, lateral depth 44 and medial depth 45would be the same.

The alternative baseplates 10′, 10″ of FIGS. 4 and 5 are provided forcontrast. Because lateral and medial fixation pegs 30′, 32′ of thealternative baseplate 10′ of FIG. 4, for example, are aligned in ananterior-posterior direction, the lateral peg distance 46′ from anteriorface 18′ to lateral fixation peg 30′ is the same as the medial pegdistance 48 from anterior face 18′ to medial fixation peg 32′. The sameis also true for lateral peg distance 46″ and medial peg distance 48″ ofthe alternative baseplate 10″ of FIG. 5. Because the alternativebaseplate 10′ of FIG. 4 has an asymmetric outer periphery 12′, medialdepth 45′ differs from lateral depth 44′. Because the alternativebaseplate 10″ of FIG. 5 has a symmetric outer periphery 12″, on theother hand, medial depth 45″ is the same as lateral depth 44″.

8. Asymmetric Positioning of Fixation Pegs for Set of Prostheses

Baseplate 10 may be provided in a kit or set of different prosthesissizes. In one embodiment, nine baseplates 10 are provided in the set,with baseplates 10 growing progressively in lateral anterior/posteriordepth 44 and/or other dimensions, for example. The progressive growth ofperiphery 12 of baseplates 10 across the set or family of baseplatesizes is described in detail in U.S. Patent Application Publication No.2012/0022660 filed Jul. 22, 2011 and entitled ASYMMETRIC TIBIALCOMPONENTS FOR A KNEE PROSTHESIS (Attorney Docket: ZIM0815-02), theentire disclosure of which is hereby expressly incorporated herein byreference.

Referring next to FIG. 6, exemplary peg distances 46, 48 are graphicallypresented for a set of prostheses of different sizes. More specifically,exemplary peg distances 46, 48 are graphically presented for a set ofprostheses having different lateral depths 44. The vertical axis of FIG.6 shows peg distances 46, 48 (in millimeters), while the horizontal axisof FIG. 6 shows various lateral depths 44 (also in millimeters) and thecorresponding nominal size indicator (1-9). The data points locatedfarther to the left represent smaller lateral depths 44 (and thereforesmaller nominal prosthesis sizes), and data points located farther tothe right represent larger lateral depths 44 (and therefore largernominal prosthesis sizes). In accordance with FIG. 2C, peg distances 46,48 and lateral depth 44 are measured posteriorly from flat portion 18 aof anterior face 18.

For each given prosthesis size (i.e., each discrete value of lateraldepth 44), a pair of points are presented for lateral and medial pegdistances 46, 48, respectively, with a space between the pair of points.This space indicates that peg distances 46, 48 are different for each ofthe nine given prosthesis sizes. Medial peg distances 48 consistentlyexceed the corresponding lateral peg distances 46 for each of the ninegiven prosthesis sizes. For example, each medial peg distance 48 mayexceed the corresponding lateral peg distance 46 by 7 mm to 11 mm. Inthis manner, each of the given prostheses has anterior/posteriorasymmetry of fixation pegs 30, 32 with respect to anterior face 18.

FIG. 6 also demonstrates that, as the prosthesis size increases, medialpeg distances 48 may increase at a faster rate than lateral pegdistances 46. In the illustrated embodiment of FIG. 6, medial pegdistances 48 increase at a rate (i.e., slope) of approximately 0.9,while lateral peg distances 46 increase at a rate of approximately 0.6.As a result, the difference between medial peg distance 48 and itscorresponding lateral peg distance 46 increases as the prosthesis sizeincreases, causing fixation pegs 30, 32 to become more and more spacedapart as the prosthesis size increases.

With respect to the alternative baseplate 10′ of FIG. 4, by contrast,where the lateral peg distance 46′ is the same as the medial pegdistance 48′, the peg distances 46′, 48′ would overlap graphically inFIG. 6. Thus, for any given prosthesis size, a single pointcorresponding to both lateral peg distance 46′ and medial peg distance48′ would be presented in FIG. 6, without a space therebetween. Also,because fixation pegs 30′, 32′ of baseplate 10′ are aligned alongmedial-lateral axis 50′, and not forward of or behind medial-lateralaxis 50′ like fixation pegs 30, 32 of baseplate 10, the overlapping pegdistances 46′, 48′ of the alternative baseplate 10′ would fall somewherebetween the spaced-apart peg distances 46, 48 of FIG. 6, The same resultwould occur with the overlapping peg distances 46″, 48″ of thealternative baseplate 10″ of FIG. 5.

According to an exemplary embodiment of the present disclosure, theabove-described distances, including inset distance 39, medial-lateralseparation distance 36, offset distance 40, anterior-posteriorseparation distance 42, lateral peg distance 46, and medial peg distance48, are measured along distal surface 34 of baseplate 10. As a result,the distances are measured near the intersection of each peg 30, 32 withdistal surface 34 (e.g., near the proximal end of each peg 30, 32). Inembodiments where pegs 30, 32 are perpendicular to distal surface 34,the distances could also be measured away from distal surface 34 (e.g.,near the distal end of each peg 30, 32) without impacting themeasurements. In embodiments where pegs 30, 32 are canted relative todistal surface 34, however, the measurements could vary if taken awayfrom distal surface 34 (e.g., near the distal end of each canted peg 30,32). Therefore, for consistency, the measurements are taken along distalsurface 34 of baseplate 10.

9. Force Testing of Asymmetric Fixation Pegs

A first prosthesis was manufactured, as shown in FIG. 7, by mountingbearing component 53 onto baseplate 10, with baseplate 10 having anasymmetric outer periphery 12 and asymmetrically arranged lateral andmedial fixation pegs 30, 32 (FIGS. 1-3). A second prosthesis (not shown)was manufactured by mounting a similar bearing component 53 onto analternative baseplate 10′, with the alternative baseplate 10′ having anasymmetric outer periphery 12′ but aligned lateral and medial fixationpegs 30′, 32′ (FIG. 4). A third prosthesis (not shown) was manufacturedby mounting a similar bearing component 53 onto another alternativebaseplate 10″, with the other alternative baseplate 10″ having asymmetric outer periphery 12″ and aligned lateral and medial fixationpegs 30″, 32″ (FIG. 5).

The illustrative bearing component 53 has lateral articular surface 54,medial articular surface 56, and spine 58 located therebetween. Whenbearing component 53 is assembled onto baseplate 10, as shown in FIG. 7,lateral articular surface 54 of bearing component 53 aligns with lateralcompartment 14 of baseplate 10, medial articular surface 56 of bearingcomponent 53 aligns with medial compartment 16 of baseplate 10, andspine 58 aligns with interior compartment 17 (FIG. 1) of baseplate 10.For the first and second prostheses, bearing component 53 had athickness T of 20 mm. For the third prosthesis, bearing component 53 hada thickness T of 17 mm. Bearing component 53 and its associatedarticular surfaces 54, 56 are described in detail in U.S. ProvisionalPatent Application Ser. No. 61/561,657, filed Nov. 18, 2011 (AttorneyDocket No. ZIM0912), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/577,293, filed Dec. 19, 2011 (AttorneyDocket No. ZIM0912-01), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/592,576, filed on Jan. 30, 2012 (AttorneyDocket No. ZIM0912-02), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/621,361 filed Apr. 6, 2012 (AttorneyDocket No, ZIM0912-03), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/621,363 filed Apr. 6, 2012 (AttorneyDocket No. ZIM0912-04), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/621,364 filed Apr. 6, 2012 (AttorneyDocket No. ZIM0912-05), and are further described in U.S. ProvisionalPatent Application Ser. No. 61/621,366 filed Apr. 6, 2012 (AttorneyDocket No, ZIM0912-06), and are further described in U.S. patentapplication Ser. No. 13/459,037 filed Apr. 27, 2012 (Attorney Docket No.ZIM0912-07), and are further described in U.S. patent application Ser.No. 13/459,041 filed Apr. 27, 2012 (Attorney Docket No. ZIM0912-08), andare further described in U.S. patent application Ser. No. 13/459,048filed Apr. 27, 2012 (Attorney Docket No. ZIM0912-09), and are furtherdescribed in U.S. patent application Ser. No. 13/459,056 filed Apr. 27,2012 (Attorney Docket No. ZIM0912-10), all entitled “TIBIAL BEARINGCOMPONENT FOR A KNEE PROSTHESIS WITH IMPROVED ARTICULARCHARACTERISTICS,” the entire disclosures of which are herebyincorporated by reference herein.

As shown in FIG. 7, a lateral compressive force F_(CL) was applied ontolateral articular surface 54 of each bearing component 53, and a medialcompressive force F_(CM) was applied onto medial articular surface 56 ofeach bearing component 53. The compressive forces F_(CL), F_(CM)measured 202 N.

Simultaneously with application of the compressive forces F_(CL),F_(CM), an anterior-facing force F_(AP) was applied to thedistal/posterior base of spine 58, as shown in FIG. 7. Theanterior-facing force F_(AP) measured 725 N for the first and secondprostheses and was scaled up to 791 N for the third prosthesis toaccount for the thinner bearing component 53.

Forces F_(CL), F_(CM), and F_(AP) were designed in magnitude and area ofapplication to replicate forces exerted on tibial bearing component 53by a prosthetic femoral component, e.g., femoral component 70, during akneeling motion. An exemplary femoral component which articulates withtibial bearing component 53 is described in U.S. Provisional PatentApplication Ser. No. 61/561,658, filed Nov. 18, 2011 (Attorney DocketNo. ZIM0915), and is further described in U.S. Provisional PatentApplication Ser. No. 61/579,873, filed Dec. 23, 2011 (Attorney DocketNo. ZIM0915-01), and is further described in U.S. Provisional PatentApplication Ser. No. 61/592,575, filed on Jan. 30, 2012 (Attorney DocketNo. ZIM0915-02), and is further described in U.S. Provisional PatentApplication Ser. No. 61/594,113 filed on Feb. 2, 2012 (Attorney Docket:ZIM0915-03), and is further described in and in U.S. Provisional PatentApplication Ser. No. (Attorney Docket: ZIM0915-04), and are furtherdescribed in U.S. Provisional Patent Application Ser. No. 61/621,372filed Apr. 6, 2012 (Attorney Docket No. ZIM0915-05), and are furtherdescribed in U.S. Provisional Patent Application Ser. No. 61/621,373filed Apr. 6, 2012 (Attorney Docket No. ZIM0915-06), and are furtherdescribed in U.S. patent application Ser. No. 13/459,061 filed Apr. 27,2012 (Attorney Docket No. ZIM0915-07), and are further described in U.S.patent application Ser. No. 13/459,064 filed Apr. 27, 2012 (AttorneyDocket No. ZIM0915-08), and are further described in U.S. patentapplication Ser. No. 13/459,060 filed Apr. 27, 2012 (Attorney Docket No.ZIM0915-09), all entitled “FEMORAL COMPONENT FOR A KNEE PROSTHESIS WITHIMPROVED ARTICULAR CHARACTERISTICS,” the entire disclosures of which arehereby expressly incorporated herein by reference.

Finite element analysis was performed on the first, second, and thirdprostheses to evaluate and compare stresses experienced at the interfaceof baseplates 10, 10′, 10″ and a simulated tibial bone that was wellfixed to each respective baseplate. Peak stresses experienced in theabove-described loading scenario were substantially reduced for thefirst baseplate 10 having asymmetrically arranged fixation pegs 30, 32as compared to the second baseplate 10′ having aligned fixation pegs30′, 32′ and the third baseplate 10″ having aligned fixation pegs 30″,32″. More particularly, a 51% reduction in peak stress was observed inthe first baseplate 10 as compared to the second baseplate 10′, and a46% reduction in peak stress was observed in the first baseplate 10 ascompared to the third baseplate 10″.

10. Additional Fixation Pegs

In addition to lateral fixation peg 30 described above, lateralcompartment 14 of tibial baseplate 100 may further include at least oneadditional lateral fixation peg 330. As shown in FIG. 13, the additionallateral fixation peg 330 is substantially centered within the PLquadrant. The illustrative lateral fixation peg 330 is positionedanteriorly/posteriorly between lateral fixation peg 30 and medialfixation peg 32, such that lateral fixation peg 30 is the anterior-mostfixation peg on tibial baseplate 100 and medial fixation peg 32 is theposterior-most fixation peg on tibial baseplate 100. As a result oflateral fixation peg 30 being medially biased toward home axis A_(H), asdescribed above, the illustrative lateral fixation peg 330 is locatedlaterally outward of lateral fixation peg 30 and is the lateral-mostfixation peg on tibial baseplate 100.

In addition to medial fixation peg 32 described above, medialcompartment 16 of tibial baseplate 100 may further include at least oneadditional medial fixation peg 332. As shown in FIG. 13, the additionalmedial fixation peg 332 is substantially centered within the AMquadrant. The illustrative medial fixation peg 332 is positionedanteriorly/posteriorly between lateral fixation peg 30 and medialfixation peg 32, such that lateral fixation peg 30 is the anterior-mostfixation peg on tibial baseplate 100 and medial fixation peg 32 is theposterior-most fixation peg on tibial baseplate 100. As a result ofmedial fixation peg 32 being medially biased away from home axis Au, asdescribed above, the illustrative medial fixation peg 332 is locatedlaterally inward of medial fixation peg 32.

11, Fixation Keel

Turning to FIGS. 8 and 9, tibial baseplate 100 is provided that issubstantially similar to baseplate 10 of FIGS. 1-3, except thatbaseplate 100 includes a single fixation structure, illustratively keel130, that extends distally from distal surface 134 and into tibia T(FIG. 1). Keel 130 may be monolithically or integrally formed as part oftibial baseplate 100, or keel 130 may be separately attachable to distalsurface 134 of tibial baseplate 100. Structures of baseplate 100 thatcorrespond to structures of baseplate 10 have corresponding referencenumerals, with the number 100 being added to the reference numerals ofbaseplate 10 to arrive at the corresponding reference numerals ofbaseplate 100, except as otherwise noted.

The illustrative keel 130 of FIG. 9A has a cylindrical core 131 defininglongitudinal axis A_(K) (i.e., the axis of the cylinder defined bycylindrical core 131) and having two or more fins 133 extending radiallyoutwardly therefrom, the fins being arranged symmetrically relative tothe cylindrical core 131. More particularly, fins 133 extend alongsubstantially all of the longitudinal extent PD_(K) (FIG. 9B) of keel130, as best shown in FIGS. 9A and 913, such that fins 133 terminate ator near the distal tip of keel 133. In an exemplary embodiment,longitudinal extent PD_(K) of tibial keel cylindrical core 131 may rangefrom 27 mm to 48 mm, with smaller nominal sizes of baseplate 100 havingrelatively lesser extents PD_(K) and larger nominal sizes of baseplate100 having relatively greater extents PD_(K).

Keel fins 133 also define keel fin angle γ with respect to longitudinalaxis A_(K) of cylindrical core 131 of keel 130. In an exemplaryembodiment, keel angle γ is equal to between 22 degrees and 27 degrees.Keel fin angle γ and longitudinal extent longitudinal extent PD_(K) ofcylindrical core 131 cooperate to define a medial/lateral keel extentML_(K) (FIG. 9B) of between 38 mm and 54 mm, with smaller nominal sizesof baseplate 100 having relatively lesser extents ML_(K) and largernominal sizes of baseplate 100 having relatively greater extents ML_(K).Advantageously, this medial/lateral extent ML_(K) defined by fins 133 ofkeel 130 present high resistance to rotation of tibial baseplate 100 invivo, and enhance the overall strength of baseplate 100.

In an exemplary embodiment, keel 130 defines a substantially cylindricalouter profile as illustrated in FIG. 9A. Where such cylindrical outerprofile is employed, an exemplary embodiment of core 131 of keel 130 maymaintain an outer diameter between 14 mm and 16 mm, with such diameterremaining constant across the longitudinal extent. However, it iscontemplated that core 131 of keel 130 may have a conical, tapered outerprofile, particularly for small-stature baseplate sizes. The taper anglemay be formed, for example, by tapering core 131 of keel 130 from acircular outer diameter of 17.1 mm at the proximal terminus of keel 130(i.e., at the junction between keel 130 and distal surface 134 of tibialbaseplate 100) to a circular diameter of 13.4 mm at the distal terminusof keel 130. An exemplary conical keel used in conjunction with asmall-stature baseplate size is disclosed in U.S. Provisional PatentApplication Ser. No. 61/592,574 filed Jan. 30, 2012 (Attorney Docket No.ZIM0919) and in U.S. Provisional Patent Application Ser. No. (AttorneyDocket No. ZIM0919-01), 61/621,374 filed Apr. 6, 2012 both entitledASYMMETRIC TIBIAL COMPONENTS FOR A KNEE PROSTHESIS, the entiredisclosures of which are hereby expressly incorporated herein byreference.

Prior art tibial baseplates include constant-diameter keels in thisdiameter range, such as the Zimmer NexGen Stemmed Tibial Plates andNatural Knee II Modular Cemented Tibial Plates. The NexGen StemmedTibial Plates and Natural Knee II Modular Cemented Tibial Plates areshown at pages 14 and 28, respectively, of the “Zimmer® TibialBaseplate, Pocket Guide United States Version.” the entire disclosure ofwhich is hereby expressly incorporated herein by reference, a copy ofwhich is submitted on even date herewith in an Information DisclosureStatement.

In FIG. 8, keel 130 is represented by a phantom oval to show the generallocation of keel 130, not necessarily the size or shape of keel 130.Rather than being cylindrical in shape, it is also within the scope ofthe present disclosure that core 131 of keel 130 may be conical inshape, with an outer diameter that tapers distally.

As discussed above, fixation pegs 30, 32 of baseplate 10 (FIGS. 1-3) maybe designed to interact with cancellous bone surrounding theintramedullary canal of the patient's tibia T. To enhance thisinteraction with the cancellous bone, fixation pegs 30, 32 may beconstructed of a highly porous biomaterial that accepts bone ingrowth.Keel 130 of baseplate 100 (FIGS. 8 and 9), by contrast, may be designedto fit into the intramedullary canal of the patient's tibia T. Likefixation pegs 30, 32, keel 130 may also be constructed of a highlyporous biomaterial that accepts bone ingrowth. Alternatively, ratherthan achieving fixation via bone ingrowth, keel 130 may be constructedof a solid metal that achieves fixation via a tight interference fitwith the patient's surrounding bone.

Although keel 130 may be the only fixation structure on baseplate 100,it is also within the scope of the present disclosure to combine keel130 with additional fixation structures. In one embodiment, keel 130 maybe combined with the above-described fixation pegs 30, 32 (FIGS. 1-3).On another embodiment, keel 130 may be combined with sharp spikes (notshown). Such spikes may be located in the same general areas discussedabove with respect to fixation pegs 30, 32. However, unlike theblunt-tipped and porous fixation pegs 30, 32, the spikes may besharp-tipped to pierce the patient's bone and may be solid inconstruction. The spikes may also have external ribs or barbs to enhancefixation with the patient's bone.

Keel 130 may also include a tapered bore (not shown) extendingproximally into the distal tip of keel 130, designed to mate with acorresponding locking-taper surface of a tibial stem extension.

12. Lateral/Medial Positioning of Fixation Keel

As shown in FIG. 9A, keel 130 is asymmetrically disposed on distalsurface 134 of baseplate 100 with respect to home axis A_(H). Moreparticularly, the longitudinal keel axis A_(K) of keel 130 is biasedmedially with respect to the vertical plane that contains home axisA_(H), i.e., keel axis A_(K) is offset toward medial compartment 116 andaway from lateral compartment 114 by offset distance 163. Throughout thefollowing paragraphs, home axis Ai and the vertical plane that containshome axis A_(H) are used interchangeably.

According to an exemplary embodiment of the present disclosure, offsetdistance 163 is measured along distal surface 134 of baseplate 100. As aresult, offset distance 163 is measured medially from the intersectionof home axis Au and distal surface 134 to the intersection of keel axisA_(K) and distal surface 134 (e.g., near the proximal end of keel 130).In embodiments where keel axis A_(K) is perpendicular to distal surface134, offset distance 163 could also be measured away from distal surface134 (e.g., near the distal end of keel 130) without impacting themeasurement. In embodiments where keel axis A_(K) is canted relative todistal surface 134, however, the measurement could vary if taken awayfrom distal surface 134 (e.g., near the distal end of the canted keel130). Therefore, for consistency, the measurement is taken along distalsurface 134 of baseplate 100.

In embodiments where baseplate 100 has a symmetric outer periphery 112,an anterior-posterior axis of symmetry through outer periphery 112 maybe used as a “home axis” A_(H) for referencing medial face 160, lateralface 162, keel 130, and other components of baseplate 100. This homeaxis A_(H) would be substantially centered between medial face 160 andlateral face 162. With keel axis A_(K) being medially offset from thecentral home axis A_(H), keel axis A_(K) would be positioned closer tomedial face 160 than lateral face 162. Thus, medial distance 164 betweenkeel axis A_(K) and the medial-most portion of medial face 160 would beless than lateral distance 166 between keel axis A_(K) and thelateral-most portion of lateral face 162.

In embodiments where baseplate 100 has an asymmetric outer periphery112, as shown in FIGS. 8 and 9, home axis A_(H) would not constitute anaxis of symmetry and would be positioned closer to lateral face 162 thanmedial face 160. Depending on the degree to which keel axis A_(K) ismedially offset from home axis A_(H), keel axis A_(K) may still bepositioned closer to medial face 160 than lateral face 162. Thus, medialdistance 164 between keel axis A_(K) and the medial-most portion ofmedial face 160 may be less than lateral distance 166 between keel axisA_(K) and the lateral-most portion of lateral face 162.

The degree of medialization of keel 130 may be expressed as a ratio or apercentage and may be calculated by dividing the offset distance 163between keel axis A_(K) and home axis A_(H) by the total medial/lateralwidth of distal surface 134 (i.e., medial distance 164 plus lateraldistance 166). For baseplate 100 having the dimensions set forth inTable 1 below, for example, the degree of medialization would beapproximately 6% (calculated as 5 mm/88 mm×100%).

TABLE 1 Sample Dimensions of a Large-Size Baseplate 100 % of TotalDimension Value (mm) Width Offset Distance 163 between Keel Axis A_(K)and 5  6 Home Axis A_(H) Medial Distance 164 41 47 Lateral Distance 16647 53 Total Width (Medial Distance 164 + Lateral 88 N/A Distance 166)

Advantageously, the medial bias of keel 130 (i.e., the relatively shortmedial distance 164 and the relatively long lateral distance 166) moreclosely aligns keel 130 with the intramedullary canal of the patient'stibia T (FIG. 1). Thus, upon implantation of baseplate 100 onto thepatient's tibia T, keel 130 may be centered or nearly centered withinthe intramedullary canal. In this manner, keel 130 may avoid impingingonto hard, cortical bone around the intramedullary canal, therebypromoting firm and stable long-term fixation of tibial baseplate 100 totibia T. The medial bias of keel 130 may also be important if it becomesnecessary to attach a distal stem extension (not shown) to keel 130,such as during a revision surgical procedure. In this manner, tibialbaseplate 100 may achieve an optimum metaphyseal fit on tibia T in theregion of keel 130 and diaphyseal fit on tibia T in the region of thedistal stem extension.

13. Lateral/Medial Positioning of Fixation Keel for Set of Prostheses

Baseplate 100 may be provided in a kit or set of different prosthesissizes. In one embodiment, nine nominal sizes of baseplate 100 areprovided in the set, with baseplates 100 growing progressively in size.

According to an exemplary embodiment of the present disclosure, thedegree of medialization of keel 130 increases as the prostheses in theset grow in size. Thus, rather than maintaining a fixed relationshipbetween medial distance 164 and lateral distance 166 as the prosthesesgrow in size, medial distance 164 makes up a smaller and smaller portionof the total width as the prostheses grow in size, and lateral distance166 makes up a larger and larger portion of the total width as theprostheses grow in size. Stated differently, the rate at which keel 130moves toward medial face 160 exceeds that rate at which the prosthesesgrow in size.

The dimensions of another sample baseplate 100 are provided in Table 2below. Baseplate 100 of Table 2, which has a total width of 58 mm, issmaller than baseplate 100 of Table 1 above, which has a total width of88 mm.

TABLE 2 Sample Dimensions of a Small-Size Baseplate 100 Dimension Value(mm) % of Total Width Offset Distance 163 between Keel Axis 1  2 A_(K)and Home Axis A_(H) Medial Distance 164 29 50 Lateral Distance 166 29 50Total Width (Medial Distance 58 N/A 164 + Lateral Distance 166)

As baseplates 100 of the present set grow in size from Table 2 to Table1 (i.e., from a small nominal size having a 58 mm total width to a largenominal size having an 88 mm total width), the degree of medializationof keel 130 increases relative to home axis A_(H) (from 2% to 6%). Also,as keel 130 moves medially from the small size of Table 2 to the largesize of Table 1, medial distance 164 makes up a smaller portion of thetotal width (from 50% to 47%), and lateral distance 166 makes up alarger portion of the total width (from 50% to 53%).

Advantageously, increasing the degree of medialization of keel 130 asbaseplate 100 grows in size may better track the position of theintramedullary canal as the patient's tibia T (FIG. 1) grows in size,Therefore, keel 130 may be positioned inside the intramedullary canalrather than in hard, cortical bone around the intramedullary canal.

The increasing medialization of keel 130 is presented graphically inFIG. 10, where exemplary offset distances 163 between keel axis A_(K)and home axis A_(H) are shown for a set of prostheses of differentsizes. More specifically, exemplary offset distances 163 between keelaxis A_(K) and home axis A_(H) are shown for a set of prostheses havingdifferent medial/lateral widths (i.e., medial distance 164 plus lateraldistance 166). The data points located farther to the left representsmaller medial/lateral widths (and therefore smaller prosthesis sizes),and data points located farther to the right represent largermedial/lateral widths (and therefore larger prosthesis sizes). Althoughadjacent nominal prosthesis sizes may share the same offset distance 163between keel axis A_(K) and home axis A_(H) (compare, for example, thecorresponding offset distances 163 of the size 5 and size 6 implants,shown as the fifth- and sixth-from left data points respectively), theoverall trend in FIG. 10 is that offset distance 163 increases as totalmedial/lateral width increases.

In a smaller bone, the metaphyseal region of tibia T is more closelyaligned with the diaphyseal region of tibia T. Therefore, keel 130 mayachieve an optimum metaphyseal and diaphyseal fit with a relativelysmall offset distance 163 (e.g., 1 mm, 2 mm). In a larger bone, bycontrast, the metaphyseal region of tibia T is more offset from thediaphyseal region of tibia T. Therefore, keel 130 may require arelatively large offset distance 163 (e.g., 4 mm, 5 mm) to achieve anoptimum metaphyseal and diaphyseal fit. FIG. 10 presents exemplaryoffset distances 163, but for any given size, offset distance 163 mayvary by +/−0.5 mm, +/−1.0 mm, +/−1.5 mm, or +/−2.0 mm, for example.

As discussed above, the degree of medialization of keel 130 may beexpressed as a percentage by dividing the offset distance 163 betweenkeel axis A_(K) and home axis A_(H) by the total medial/lateral width.In FIG. 11, the offset distances 163 from FIG. 10 are shown aspercentages of the total medial/lateral width. The overall trend in FIG.11 is that the degree of medialization of keel 130 increases asmedial/lateral width increases. With respect to a relatively smallnominal size 3 implant, for example, the medial offset of keel 130 fromhome axis A_(H) is 3% of the total medial/lateral implant width. Withrespect to a relatively large nominal size 7 implant, the medial offsetof keel 130 from home axis A_(H) is 5% of the total medial/lateralimplant width.

14. Anterior/Posterior Positioning of Fixation Keel for Set ofProstheses

As shown in FIG. 8, the anterior/posterior keel distance 147 may bemeasured posteriorly from flat portion 118 a of anterior face 118 tokeel axis A_(K), for example. The lateral depth 144 of lateralcompartment 114 is also shown being measured posteriorly from flatportion 118 a of anterior face 118 to posterior/lateral face 120 ofbaseplate 100 in FIG. 8, and this lateral depth 144 exceeds keeldistance 147.

According to an exemplary embodiment of the present disclosure, keeldistance 147 is measured along distal surface 134 of baseplate 100. As aresult, keel distance 147 is measured posteriorly from the intersectionof flat portion 18 a of anterior face 118 and distal surface 134 to theintersection of keel axis AK and distal surface 134 (e.g., near theproximal end of keel 130). In embodiments where keel axis A_(K) isperpendicular to distal surface 134, keel distance 147 could also bemeasured away from distal surface 134 (e.g., near the distal end of keel130) without impacting the measurement. In embodiments where keel axisA_(K) is canted relative to distal surface 134, however, the measurementcould vary if taken away from distal surface 134 (e.g., near the distalend of the canted keel 130). Therefore, for consistency, the measurementis taken along distal surface 134 of baseplate 100.

Across a set of different tibial baseplates 100 having varying nominalsizes, the anterior/posterior positioning of keel 130 may vary. In FIG.12, for example, exemplary anterior/posterior keel distances 147 areshown for a set of prostheses of different sizes. The overall trend inFIG. 12 is that keel distance 147 increases as lateral depth 144increases. Moving keel 130 further and further from anterior face 118 asbaseplate 100 increases in size may avoid anterior cortical boneimpingement by keel 130, especially if keel 130 also increases in size(e.g., diameter, length) along with baseplate 100. FIG. 12 depictsexemplary keel distances 147, but for any given size, keel distance 147may vary by +/−0.5 mm, +/−1.0 mm, +/−1.5 mm, or −/−2.0 mm, for example.

15. Proximal Keel Expansion

As shown in FIG. 9A, the illustrative keel 130 includes a blind proximalbore 180 therein that is sized to receive a fixation structure, such asa set screw (not shown), from proximal surface 111 of baseplate 100. Thefixation structure may be used to attach a tibial bearing component ontoproximal surface 111 of baseplate 100, for example.

The illustrative bore 180 of FIG. 9A is centered along home axis A_(H).However, because keel axis A_(K) is offset from home axis A, bore 180becomes offset in keel 130. To ensure that the walls of keel 130surrounding bore 180 are adequately thick along the axial extent of bore180 (e.g., 1.5 mm), keel 130 may expand radially outwardly around bore180 to form bulge 182.

As keel 130 becomes more and more offset from home axis A_(H) and bore180, bulge 182 may become larger and larger in size, For example, formedium nominal prosthesis sizes (e.g., sizes 5 and 6) having mediumoffset distances 163 between keel axis A_(K) and home axis A_(H) (e.g.,3 mm), bulge 182 may increase the diameter of keel 130 by 0.5 mm. Forlarge nominal prosthesis sizes (e.g., sizes 7-9) having large offsetdistances 163 between keel axis A_(K) and home axis Au (e.g., 4 mm, 5mm), bulge 182 may increase the diameter of keel 130 by 1.4 mm. Forsmall nominal prosthesis sizes (e.g., sizes 1-4) having small offsetdistances 163 between keel axis A_(K) and home axis A_(H) (e.g., 1 mm, 2mm), bulge 182 may be excluded.

While this invention has been described as having exemplary designs, thepresent invention can be further modified within the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. A tibial prosthesis system comprising: a firsttibial baseplate comprising: a first proximal surface; a first distalsurface opposite the first proximal surface, the first distal surfacesized and shaped to substantially cover a proximal resected surface of atibia; a first medial face; a first lateral face opposite the firstmedial face; a first total width measured from the first medial face tothe first lateral face; and a first keel extending distally from thefirst distal surface, the first keel spaced from the first medial faceby a first medial distance and spaced apart from the first lateral faceby a first lateral distance; and a second tibial baseplate comprising: asecond proximal surface; a second distal surface opposite the secondproximal surface, the second distal surface sized and shaped tosubstantially cover a proximal resected surface of a tibia; a secondmedial face; a second lateral face opposite the second medial face; asecond total width measured between the second medial face and thesecond lateral face, the second total width differing from the firsttotal width whereby the first and second tibial baseplates compriseunique nominal sizes; and a second keel extending distally from thesecond distal surface, the second keel spaced apart from the secondmedial face by a second medial distance and spaced apart from the secondlateral face by a second lateral distance, a first ratio of the firstmedial distance to the first total width differing from a second ratioof the second medial distance to the second total width.
 2. The tibialprosthesis system of claim 1, wherein the first total width is greaterthan the second total width, and the first ratio is less than the secondratio.
 3. The tibial prosthesis system of claim 1, wherein: the firstkeel is medially biased toward the first medial face, such that thefirst lateral distance exceeds the first medial distance; and the secondkeel is medially biased toward the second medial face, such that thesecond lateral distance exceeds the second medial distance.
 4. Thetibial prosthesis system of claim 1, wherein: the first tibial baseplateincludes: a first medial compartment; a first lateral compartmentopposite the first medial compartment; and a first, anterior-posterioraxis extending centrally between the first medial and lateralcompartments throughout its length, the first keel being medially offsetfrom the first axis by a first offset distance; and the second tibialbaseplate includes: a second medial compartment; a second lateralcompartment opposite the second medial compartment; and a second,anterior-posterior axis extending centrally between the second medialand lateral compartments throughout its length, the second keel beingmedially offset from the second axis by a second offset distance lessthan the first offset distance.
 5. The tibial prosthesis system of claim4, wherein the first total width is greater than the second total width,and wherein the first offset distance constitutes a greater portion ofthe first total width than the second offset distance constitutes of thesecond total width.
 6. The tibial prosthesis system of claim 4, whereinthe first axis and the second axis are positioned to correspond to ahome axis of a patient's proximal tibia when one of the first and secondtibial baseplates, respectively, is implanted onto the patient'sproximal tibia, the home axis defined as a line extending from: aposterior point disposed at a geometric center of an attachment areabetween a posterior cruciate ligament and the patient's proximal tibia,to an anterior point disposed on an anterior tubercle of the patient'sproximal tibia and bordering a medial third of the anterior tubercle. 7.The tibial prosthesis system of claim 4, wherein the first and secondtibial baseplates are symmetric about the first axis and second axis,respectively.
 8. The tibial prosthesis system of claim 4, wherein thefirst and second tibial baseplates are asymmetric about the first axisand second axis, respectively.
 9. The tibial prosthesis system of claim4, wherein the first total width is greater than the second total width,the first keel having a first bulge around a first proximal bore formedin the first tibial baseplate, and the second keel having a second bulgearound a second proximal bore formed in the second tibial baseplate,wherein the first bulge is larger than the second bulge.
 10. The tibialprosthesis system of claim 4, wherein the first tibial baseplate islarger than the second tibial baseplate, and wherein: the first tibialbaseplate includes a first anterior face joining the first lateral faceto the first medial face, the first keel being posteriorly spaced fromthe first anterior face by a first distance; and the second tibialbaseplate includes a second anterior face joining the second lateralface to the second medial face, the second keel being posteriorly spacedfrom the second anterior face by a second distance less than the firstdistance.
 11. The tibial prosthesis system of claim 1, wherein at leastone of the first keel and the second keel defines a longitudinal extentequal to between 27 mm and 48 mm.
 12. The tibial prosthesis system ofclaim 1, further comprising a tibial bearing component configured to bereceived on at least one of the first proximal surface and the secondproximal surface in one of a fixed relationship and a slidingrelationship, the tibial bearing component comprising: a lateralarticular surface positioned to align with a lateral compartment of oneof the first tibial baseplate and the second tibial baseplate; and amedial articular surface positioned to align with a medial compartmentof one of the first tibial baseplate and the second tibial baseplate.13. The tibial prosthesis system of claim 12, wherein the medial andlateral articular surfaces of the tibial bearing component are shaped toarticulate with medial and lateral femoral condyles respectively. 14.The tibial prosthesis system of claim 1, wherein each of the first keeland the second keel comprises a cylindrical core defining a longitudinalkeel axis.
 15. The tibial prosthesis system of claim 14, wherein thecylindrical core of each of the first keel and the second keel defines adiameter of at least 14 mm.
 16. The tibial prosthesis system of claim14, wherein the cylindrical core defines a longitudinal extent ofbetween 27 mm and 48 mm.
 17. The tibial prosthesis system of claim 1,wherein each of the first tibial baseplate and the second tibialbaseplate comprises at least two fins extending radially outwardly froma core of the first or second keel respectively, the fins arrangedsymmetrically with respect to the core of the first or second keelrespectively.
 18. The tibial prosthesis system of claim 17, wherein eachof the fins extend along substantially of a longitudinal extent of thekeel.
 19. A tibial baseplate configured for implantation upon apatient's proximal tibia, the tibial baseplate comprising: a medialcompartment; a lateral compartment opposite the medial compartment; aproximal surface; a distal surface opposite the proximal surface, thedistal surface sized and shaped to substantially cover the patient'sproximal tibia; an outer periphery cooperatively defined by an anteriorface, a medial face, a lateral face, and at least one posterior face; afirst, anterior-posterior axis located between the medial face and thelateral face and intersecting the anterior face, the first axisextending centrally between the medial and lateral compartmentsthroughout its length; a plurality of fixation pegs extending distallyfrom the distal surface, each of the plurality of fixation pegs beingpositioned inward of the outer periphery for implantation into thepatient's proximal tibia, the plurality of fixation pegs comprising: amedial fixation peg located at the medial compartment; and a lateralfixation peg located at the lateral compartment, the lateral fixationpeg being positioned more anteriorly than each other fixation peg amongthe plurality of fixation pegs.
 20. The tibial baseplate of claim 19,wherein the plurality of fixation pegs are medially biased from thefirst axis, such that the lateral fixation peg is closer to the firstaxis than the medial fixation peg.
 21. The tibial baseplate of claim 20,wherein the plurality of fixation pegs are medially biased from thefirst axis by between 3 mm and 6 mm.
 22. The tibial baseplate of claim19, wherein the tibial baseplate further comprises a second,medial-lateral axis perpendicular to the first axis, the second axisintersecting the medial face and the lateral face, the medial fixationpeg posteriorly offset from the second axis and the lateral fixation peganteriorly offset from the second axis.
 23. The tibial baseplate ofclaim 22, wherein the second axis intersects the first axis at alocation posterior of the anterior face and anterior of a posteriorcutout formed in the at least one posterior face of the tibialbaseplate, the first and second axes intersecting within the outerperiphery of the tibial baseplate.
 24. The tibial baseplate of claim 19,wherein: the first and second axes divide the tibial baseplate into ananterior/medial quadrant, a posterior/medial quadrant, ananterior/lateral quadrant, and a posterior/lateral quadrant, the medialfixation peg is arranged more in the posterior/medial quadrant than theanterior/medial quadrant, and the lateral fixation peg is arranged morein the anterior/lateral quadrant than the posterior/lateral quadrant.25. The tibial baseplate of claim 19, wherein: the first and second axesdivide the tibial baseplate into an anterior/medial quadrant, aposterior/medial quadrant, an anterior/lateral quadrant, and aposterior/lateral quadrant, and the plurality of fixation pegs aregenerally arranged along a third axis that diagonally intersects boththe first and second axes, the third axis traveling from theanterior/lateral quadrant to the posterior/medial quadrant.
 26. Thetibial baseplate of claim 19, wherein the first axis is positioned tocorrespond to a home axis of the patient's proximal tibia when thetibial baseplate is implanted onto the patient's proximal tibia, thehome axis defined as a line extending from: a posterior point disposedat a geometric center of an attachment area between a posterior cruciateligament and the patient's proximal tibia, to an anterior point disposedon an anterior tubercle of the patient's proximal tibia and bordering amedial third of the anterior tubercle.
 27. The tibial baseplate of claim19, wherein the first axis bisects a flat portion of the anterior facein a perpendicular manner.
 28. The tibial baseplate of claim 19, whereinthe first axis bisects a posterior cutout formed in the at least oneposterior face of the tibial baseplate.
 29. The tibial baseplate ofclaim 19, wherein the outer periphery of the tibial baseplate issymmetric about the first axis.
 30. The tibial baseplate of claim 19,wherein the outer periphery of the tibial baseplate is asymmetric aboutthe first axis.
 31. The tibial baseplate of claim 30, wherein the medialcompartment has a medial anterior/posterior depth measured posteriorlyfrom the anterior face and the lateral compartment has a lateralanterior/posterior depth measured posteriorly from the anterior face,the medial depth exceeding the lateral depth.
 32. The tibial baseplateof claim 30, wherein the first axis is closer to a lateral-most point onthe lateral face than to a medial-most point on the medial face.
 33. Thetibial baseplate of claim 19, in combination with a tibial bearingcomponent configured to be received on the proximal surface of thetibial baseplate in one of a fixed relationship and a slidingrelationship, the tibial bearing component comprising: a lateralarticular surface positioned to align with a lateral compartment of thetibial baseplate; and a medial articular surface positioned to alignwith a medial compartment of the tibial baseplate.
 34. The tibialprosthesis system of claim 33, wherein the medial and lateral articularsurfaces of the tibial bearing component are shaped to articulate withmedial and lateral femoral condyles respectively.
 35. A tibial baseplateconfigured for implantation upon a patient's proximal tibia, the tibialbaseplate comprising: a medial compartment; a lateral compartmentopposite the medial compartment; a proximal surface; a distal surfaceopposite the proximal surface, the distal surface sized and shaped tosubstantially cover the patient's proximal tibia; an outer peripherycooperatively defined by an anterior face, a medial face, a lateralface, and at least one posterior face; at most one medial fixation pegassociated with the medial compartment, the medial fixation pegextending distally from the distal surface and positioned forimplantation into the patient's proximal tibia; and at most one lateralfixation peg associated with the lateral compartment, the lateralfixation peg extending distally from the distal surface and positionedfor implantation into the patient's proximal tibia, the lateral fixationpeg being located closer to the anterior face than the medial fixationpeg.
 36. The tibial baseplate of claim 35, wherein the anterior face hasa flat portion disposed between the medial and lateral compartments, thelateral fixation peg being located closer to the flat portion of theanterior face than the medial fixation peg.
 37. The tibial baseplate ofclaim 36, further comprising an anterior-posterior axis that bisects theflat portion of the anterior face and bisects a posterior cutout formedin the at least one posterior face of the tibial baseplate.
 38. Thetibial baseplate of claim 37, wherein the medial and lateral fixationpegs are medially biased from the anterior-posterior axis, such that thelateral fixation peg is closer to the anterior-posterior axis than themedial fixation peg.
 39. The tibial baseplate of claim 38, wherein themedial and lateral fixation pegs are medially biased from theanterior-posterior axis by between 3 mm and 6 mm.
 40. The tibialbaseplate of claim 35, wherein the medial and lateral fixation pegsdefine an anterior-posterior gap therebetween.
 41. The tibial baseplateof claim 40, wherein the anterior-posterior gap is between 5 mm and 11mm, as measured on center between the medial and lateral fixation pegs.42. The tibial baseplate of claim 35, in combination with a tibialbearing component configured to be received on the proximal surface ofthe tibial baseplate in one of a fixed relationship and a slidingrelationship, the tibial bearing component comprising: a lateralarticular surface positioned to align with the lateral compartment ofthe tibial baseplate when the tibial bearing component is received onthe proximal surface of the tibial baseplate; and a medial articularsurface positioned to align with the medial compartment of the tibialbaseplate when the tibial bearing component is received on the proximalsurface of the tibial baseplate.
 43. The tibial prosthesis system ofclaim 42, wherein the medial and lateral articular surfaces of thetibial bearing component are shaped to articulate with medial andlateral femoral condyles respectively.
 44. A tibial baseplate configuredfor implantation upon a patient's proximal tibia, the tibial baseplatecomprising: a medial compartment; a lateral compartment opposite themedial compartment; a proximal surface; a distal surface opposite theproximal surface, the distal surface sized and shaped to substantiallycover the patient's proximal tibia; an outer periphery cooperativelydefined by an anterior face, a medial face, a lateral face, and at leastone posterior face; a first, anterior-posterior axis located between themedial face and the lateral face and intersecting the anterior face, thefirst axis extending centrally between the medial and lateralcompartments throughout its length; a first fixation peg extendingdistally from the distal surface, the first fixation peg being insetfrom the outer periphery for implantation into the patient's proximaltibia, the first fixation peg being medially spaced from the first axisby a first distance; and a second fixation peg extending distally fromthe distal surface, the second fixation peg being inset from the outerperiphery for implantation into the patient's proximal tibia, the secondfixation peg being laterally spaced from the first axis by a seconddistance, the second distance less than the first distance.
 45. Thetibial baseplate of claim 44, wherein the first fixation peg is the onlyfixation peg associated with the medial compartment and the secondfixation peg is the only fixation peg associated with the lateralcompartment.
 46. The tibial baseplate of claim 44, wherein the first andsecond fixation pegs are the same shape.
 47. The tibial baseplate ofclaim 44, wherein: the tibial baseplate further comprises a second,medial-lateral axis perpendicular to the first axis, the second axisintersecting the medial face and the lateral face, and the second axisextends through an anterior-posterior gap formed between the first andsecond fixation pegs, such that the first fixation peg is locatedposteriorly of the second axis and the second fixation peg is locatedanteriorly of the second axis.
 48. The tibial baseplate of claim 47,wherein the anterior-posterior gap is between 5 mm and 11 mm, asmeasured on center between the first and second fixation pegs.
 49. Thetibial baseplate of claim 47, wherein the tibial baseplate comprises afirst tibial baseplate in combination with a second tibial baseplatelarger in size than the first tibial baseplate, the second tibialbaseplate having a first fixation peg and a second fixation peg thatdefine a larger anterior-posterior gap therebetween as compared to theanterior-posterior gap formed between the first and second fixation pegsof the first tibial baseplate.
 50. The tibial baseplate of claim 44,wherein the first axis is positioned to correspond to a home axis of thepatient's proximal tibia when the tibial baseplate is implanted onto thepatient's proximal tibia the home axis defined as a line extending from:a posterior point disposed at a geometric center of an attachment areabetween a posterior cruciate ligament and the patient's proximal tibia,to an anterior point disposed on an anterior tubercle of the patient'sproximal tibia and bordering a medial third of the anterior tubercle.51. The tibial baseplate of claim 44, wherein the first axis bisects aflat portion of the anterior face in a perpendicular manner.
 52. Thetibial baseplate of claim 44, wherein the first axis bisects a posteriorcutout formed in the at least one posterior face of the tibialbaseplate.
 53. The tibial baseplate of claim 44, wherein the outerperiphery of the tibial baseplate is symmetric about the first axis. 54.The tibial baseplate of claim 44, wherein the outer periphery of thetibial baseplate is asymmetric about the first axis.
 55. The tibialbaseplate of claim 44, in combination with a tibial bearing componentconfigured to be received on the proximal surface of the tibialbaseplate in one of a fixed relationship and a sliding relationship, thetibial bearing component comprising: a lateral articular surfacepositioned to align with the lateral compartment of the tibial baseplatewhen the tibial bearing component is received on the proximal surface ofthe tibial baseplate; and a medial articular surface positioned to alignwith the medial compartment of the tibial baseplate when the tibialbearing component is received on the proximal surface of the tibialbaseplate.
 56. The tibial prosthesis system of claim 55, wherein themedial and lateral articular surfaces of the tibial bearing componentare shaped to articulate with medial and lateral femoral condylesrespectively.